Three-dimensional ultrastructural analyses of anterior pituitary gland expose spatial relationships between endocrine cell secretory granule localization and capillary distribution.

Munetake Yoshitomi,Shingo Hirashima,Keisuke Ohta,Kei-Ichiro Nakamura,Motohiro Morioka,Kei-Ichiro Uemura,Akinobu Togo,Satoko Okayama,Tomonoshin Kanazawa

doi:10.1038/srep36019

Abstract

Endocrine and endothelial cells of the anterior pituitary gland frequently make close appositions or contacts, and the secretory granules of each endocrine cell tend to accumulate at the perivascular regions, which is generally considered to facilitate secretory functions of these cells. However, three-dimensional relationships between the localization pattern of secretory granules and blood vessels are not fully understood. To define and characterize these spatial relationships, we used scanning electron microscopy (SEM) three-dimensional reconstruction method based on focused ion-beam slicing and scanning electron microscopy (FIB/SEM). Full three-dimensional cellular architectures of the anterior pituitary tissue at ultrastructural resolution revealed that about 70% of endocrine cells were in apposition to the endothelial cells, while almost 30% of endocrine cells were entirely isolated from perivascular space in the tissue. Our three-dimensional analyses also visualized the distribution pattern of secretory granules in individual endocrine cells, showing an accumulation of secretory granules in regions in close apposition to the blood vessels in many cases. However, secretory granules in cells isolated from the perivascular region tended to distribute uniformly in the cytoplasm of these cells. These data suggest that the cellular interactions between the endocrine and endothelial cells promote an uneven cytoplasmic distribution of the secretory granules.

Highlights

Endocrine and endothelial cells of the anterior pituitary gland frequently make close appositions or contacts, and the secretory granules of each endocrine cell tend to accumulate at the perivascular regions, which is generally considered to facilitate secretory functions of these cells
Some types of endocrine cells are known to extend their cellular process in 3D, but such cellular processes are only observed as a fragment of the cytoplasm in transmission electron microscopy (TEM) photomicrographs
Each block face image had sufficient resolution to characterize secretory granules (SG) (Fig. 1c) and distribution pattern of the SGs, but the reconstruction was not sufficient to analyse the shape of each SG, especially in the case of Type 1 cells, because the size of the SG was equivalent to almost 2 pixels of the image

Summary

Introduction

Endocrine and endothelial cells of the anterior pituitary gland frequently make close appositions or contacts, and the secretory granules of each endocrine cell tend to accumulate at the perivascular regions, which is generally considered to facilitate secretory functions of these cells. The hormonal signals from hypothalamus to the pituitary and/or the pituitary to its target organs are thought to diffuse in the intercellular space between endothelial cell and endocrine cells[1] In this context, the spatial relationship between the capillaries and the endocrine cells plays an important role regulating their secretory function. Immunohistochemistry using anti-hormone antibodies can be used to visualize the localization of secretory granules (SG) and has frequently been used to visualize the shape of endocrine cells in pituitary gland, but the SGs are not distributed uniformly in the cytoplasms and it is not always possible to visualize all cellular processes using this method. We evaluated the spatial relationship between SG accumulation sites and the apposition between the endocrine and endothelial cells

Methods

Results

Conclusion